For WVTR measurement, moisture sensor is one of the critical components which determine the accuracy of the testing results
The phosphorus pentoxide (P2O5) moisture sensor consists of a dual platinum winding formed around a quartz tube about 8cm long. The bare platinum electrodes are coated with a thin film of P2O5. A constant DC voltage is applied across platinum electrodes during the measurement.
As the gas flows through the cell, moisture in the gas stream is attracted to the P2O5 coating and migrates through P2O5 film to the platinum electrodes (platinum wires). Electrolysis reaction of H2O into H2 and O2 occurs and current is generated accordingly which is proportional to the concentration of moisture in the carrying gas, assuming the reaction can be completed, as shown in the following equation.
Sources: systechillinois.com and testingmachines.com